1. Field of the Invention
The invention relates to a method for scheduling radar transmissions for a phased array antenna system comprising at least two antenna faces where, after a radar transmission, each antenna face receives echoes, originating from that radar transmission, of possible targets and where, on the basis of a list of requested radar transmissions, an antenna face and a starting moment are determined for at least substantially each radar transmission.
2. Discussion of the Background
Phased array antenna systems as such are well-known in the art and are for instance used on board naval ships for a considerable number of tasks which, until recently, required the availability of several separate fire-control and surveillance radar systems.
With multi-face phased array antennas, it is preferably assumed that radar transmissions for the various antenna faces can be scheduled independently. In this respect, a more specific assumption is that transmission by one antenna face and reception by another antenna face may coincide, or worded differently, that the insulation between the various antenna faces is satisfactory. Broadly speaking, this is true: it is possible to design the antenna faces such that the side lobe level is kept to a minimum, thus providing a sufficient insulation for any combination of beam directions.
The installation of the phased array antenna system on board a ship may alter the situation. If a large object, a ferry-boat for instance, is irradiated by two different antenna faces simultaneously, which is possible, as beams generated by two adjacent antenna faces will generally overlap to a slight extent, the infeasibility of coincident transmission and reception can be readily ascertained. This problem may also arise for less powerful reflectors and can be easily solved by scheduling the transmissions for adjacent antenna faces in such a manner that the beams are not parallel or not substantially parallel and by stipulating a predetermined frequency difference for transmissions for adjacent antenna faces. Notwithstanding these measures, the antenna face side lobes and the presence of a powerful reflector in the vicinity of the antenna system are still found to cause crosstalk from one antenna face to another.
An ineffectual way to solve this problem is to use only one antenna face at a time. This restricts the number of tasks to be performed by the antenna system to such an extent that the concept should no longer be considered viable.
The present invention provides a solution to the problem without any noticeable degradation of the performance of the antenna system and is characterized in that the scheduling process is arranged such that the antenna faces are used at least substantially simultaneously and that transmission by an antenna face at the moment that another antenna face is about to receive echoes of possible targets is prevented.
According to a first embodiment of the invention, the priorities assigned to the radar transmissions may vary, which may be convenient in the event of crosstalk or imminent crosstalk. It is thereto characterized in that an antenna face engaged in a high-priority radar transmission may interrupt a lower-priority radar transmission performed by an adjacent antenna face at the moment that echoes of possible targets are anticipated.
Crosstalk or imminent crosstalk may for instance be ascertained with the aid of a correlator, as described in the Netherlands patent specification 1006812, which patent specification herewith is incorporated by reference.
An advantageous embodiment of the inventive method according to which it is not required to know the positions of possible targets is characterized in that radar transmissions for the various antenna faces take place at least substantially mutually synchronized.
Another advantageous embodiment of the inventive method, which all the time ensures a maximum available listening time is characterized in that radar transmissions for the various antenna faces are scheduled such that the radar transmissions end at least substantially simultaneously.
Yet another advantageous embodiment of the method is based on the observation that a requested radar transmission has a certain degree of flexibility. It is not always necessary for the pulse repetition frequency, the PRF, to be chosen accurately, it will often suffice to stipulate that a PRF is contained in a certain frequency interval. Sometimes there may even be several frequency intervals that are suitable. The same applies to the radar transmit frequency, the RF. Although the RP is often chosen dependent on the PRF with a view to Doppler processing, only a limited number of values is found to be unsuitable. Also the moment on which a transmission is to take place is not entirely fixed, although in general there is an expiration time: the moment before which the transmission must be completed. This inventive embodiment of the invention is therefore characterized in that each requested radar transmission is represented by a realization space which contains at least substantially all acceptable realizations of the transmission. A priority assigned to the transmission and an expiration time have been determined unequivocally and a large number of possible PRF, RF pairs are available.
Yet another embodiment of the invention is characterized in that per antenna face, a group of requested radar transmissions, intended for that antenna face, is selected from the list of requested radar transmissions on the basis of their priorities and that per group, the realizations of the requested radar transmissions are scheduled such that the requested radar transmissions can for all antenna faces be realized with an identical PRF at least substantially at any moment.
It frequently occurs that a requested radar transmission can be realized by more than one antenna face, for instance because the range in azimuth of two adjacent antenna plates shows a certain degree of overlap. Usually, one realization of the requested radar transmission is noticeably superior, sometimes, however, both realizations are acceptable. Yet another embodiment of the invention makes use of this possibility and is characterized in that, if a group contains a radar transmission that can also be realized by another antenna face, this radar transmission is included in the group pertaining to the other antenna face after which, per group, the realizations of the requested radar transmissions are scheduled such that the requested radar transmissions can for all antenna faces again be realized with an identical PRF at least substantially at any moment.
Generally, a number of feasible time schedules will thus be generated, all of which are in fact suitable. Another advantageous embodiment is characterized in that from the variety of possible schedules, the schedule that ensures the most optimal utilization of all antenna faces is always selected.
Once all requested radar transmissions have been assigned to groups thus scheduled, the groups are written into a buffer memory for further processing.
A drawback in scheduling the groups is the absence of a good scheduling strategy. Consequently, an absolutely optimal schedule is never obtained. A longer search procedure will however yield a more feasible schedule. Yet another advantageous embodiment of the invention is characterized in that the generation of possible schedules is ceased when the buffer memory is at least substantially empty and that the best feasible schedule is selected from the schedules which are available at that moment.